The invention provides a glenoid shoulder implant, a humeral implant, and devices for preparing the glenoid and humeral head for joint replacement.
Shoulder replacement surgery is currently used to treat patients suffering from disabling pain due to worn or damaged shoulder joints, which can be caused by, e.g., arthritis or injury. The humeral implants currently in use are typically made from metal, and the implants are affixed to the bone using bone cement (e.g., polymethylmethacrylate) or by press fitting the implant into the bone using a roughened outer surface coating on the metal for bony integration. Most glenoid (shoulder socket) implants are made completely from polyethylene and affixed to the cortical bone using bone cement. Some glenoid implants have a rigid base plate made of metal, ceramic or rigid polymer with a polyethylene insert. The polyethylene material is suitable as a low friction articulating surface for engaging the humeral component. Current glenoid implants are intended to sit on a prepared surface of a glenoid bone. The surface is typically prepared by removing any remaining cartilage, reaming a smooth bony surface and by drilling receiving pockets for anchoring features or devices within the natural glenoid area. Current implant designs use either a keel or multiple elongated pegs on the back (medial surface) of the prosthetic glenoid implant as anchoring features to secure the glenoid implant inside the glenoid vault.
Glenoid implants with keeled or elongated peg anchors suffer from several disadvantages, which limit their lifespan once implanted and reduce the number of indications for which they can be used. For example, these glenoid implants can loosen due to poor fixation to the bone, and are prone to wear and fatigue failure of the polyethylene due to adhesion, abrasion, and shear stress. Because of these deficiencies, surgeons hesitate to perform glenoid replacement surgery on young or middle aged patients with glenoid articular cartilage injuries or damage due to early arthritis for fear that the implant may not last more than 10-15 years in the body, thus subjecting the patient to the possibility of two or more surgeries during the lifetime of the patient to preserve the function and pain-free state of the joint. Finally, current glenoid implants with a long keel or an elongated anchor peg are sometimes contraindicated in patients with significant glenoid bone loss. As arthritis progresses, the humeral head can wear medially and destroy the foundation of glenoid bone. In these cases, the glenoid vault can be significantly reduced in volume and depth. Thus, a typical keel or peg design can penetrate through the glenoid vault and injure the suprascapular nerve along the suprascapular notch or spinoglenoid notch with resultant denervation injury to the rotator cuff muscles. Penetrating through the bone of the glenoid vault can also fracture the body of the scapula and cause early implant loosening.
There are also several disadvantages associated with current glenoid replacement surgical techniques. Current techniques require extensive shoulder exposure with capsular releases in order to fully expose the glenoid surface circumferentially. Since the axillary nerve is located within 1 cm of the inferior capsule, there is potential risk of axillary nerve injury with resultant denervation injury to the deltoid muscle when these releases are performed. Use of the current glenoid implants with keels or elongated anchor pegs requires this extensive glenoid exposure for proper fitting and placement of the prostheses. Current glenoid replacement surgery also requires a long skin incision, typically 150 mm to 200 mm in length, and extensive soft tissue stripping in order to fully expose the glenoid circumferentially, which increases the risk of tissue damage and produces a cosmetically unappealing scar. Finally, current glenoid replacement surgical techniques require advanced surgical training and expertise within the specialty of shoulder surgery, yet the majority of shoulder implants performed in the U.S. every year are performed by orthopedic surgeons who do not have advanced training in the subspecialty of shoulder surgery. Therefore, many surgeons have difficulty preparing the glenoid site for a total shoulder replacement using the current techniques.
As a consequence of the limitations of the currently available designs and surgical techniques some patients forego surgery and incur a risk of continued pain and disability. Patients who elect a surgical solution incur the risk of neurovascular injuries, glenoid and scapula fractures, and failed shoulder prostheses requiring revision surgery. Thus, there remains a need for an improved glenoid implant and improved methods for performing replacement shoulder surgery.